In pumping systems where the flow demand changes over time there are several conventional procedures to adapt the operation of the pump(s) to satisfy such demand without exceeding the pressure rating of the system, burning seals or creating vibration, and they may also attempt to optimize the energy use.
Traditional systems have used one or several constant speed pumps and attempted to maintain the discharge pressure constant, when the flow demand changed, by changing the number of running pumps and/or by operating pressure reducing, bypass and discharge valves.
One popular system in use today has several pumps; each equipped with an electronic variable speed drive, and operates them to control one or more pressure(s) remotely in the system, measured by remote sensors (usually installed at the furthest location served or ⅔ down the line). At the remote sensor location(s) a minimum pressure has to be maintained, so the deviation of the measured pressure(s) with respect to the target(s) is calculated. The speed of the running pumps is then adjusted (up or down) to the lowest that maintains all the measured pressures at or above their targets. When the speed of the running pumps exceeds a certain value (usually 95% of the maximum speed), another pump is started. When the speed falls below a certain value (50% or higher, and sometimes dependent on the number of pumps running), a pump is stopped. This sequencing method is designed to minimize the number of pumps used to provide the required amount of flow.
An alternative to this type of system measures the flow and pressure at the pump(s) and estimates the remote pressure by calculating the pressure drop in the pipes in between. The pump(s) are then controlled as per the procedure described above, but using the estimated remote pressure instead of direct measurements. This alternative saves the cost of the remote sensor(s), plus their wiring and installation, but requires a local pressure sensor and flow meter.
One type of pump device estimates the local flow and/or pressure from the electrical variables provided by the electronic variable speed drive. This technology is typically referred to in the art as “sensorless pumps” or “observable pumps”. Example implementations using a single pump are described in WO 2005/064167, U.S. Pat Nos. 7,945,411, 6,592,340 and DE19618462. The single device can then be controlled, but using the estimated local pressure and flow to then infer the remote pressure, instead of direct fluid measurements. This method saves the cost of sensors and their wiring and installation, however, these references may be limited to the use of a single pump.
Another such application, where multiple pumps are coordinated to each primarily satisfy a specific corresponding load for each pump, is described in U.S. 2010/0300540.
Additional difficulties with existing systems may be appreciated in view of the description below.